This application claims the priority of German Patent Application, Serial No. 101 17 460.8, filed Apr. 6, 2001, the subject matter of which is incorporated herein by reference.
The invention relates to a drive train for a machine tool or useful load, and more particular to a drive train with two drive elements, wherein one drive element is moveably supported on a machine base.
Many industrial production processes employ machines that move workpieces and/or tools along defined pathways in axial directions. The operating speed of these machines depends on the velocity of both the feed motion and transport motion. Accordingly, a high axial velocity is desirable for achieving a high machine productivity.
Most frequently, precisely controlled electrical servo drives provide the feed motion. Unless these drives are direct drives, various drive elements, such as ball roller spindles, toothed belts or toothed racks, are used to transmit the torque of the servo motor to the machine carriage which moves in a linear direction. Such drive systems are widely used in industrial applications.
High linear velocities are inevitably accompanied by large accelerations and noticeable jolts (sudden change in the acceleration per unit time). In particular, jolts become stronger with increasing operating speed along the servo axis. The jolts induce oscillations in the machine structure which adversely affect the precision and the contour accuracy of the manufacturing process. It is therefore frequently necessary to reduce the acceleration and the axial speed so as not to exceed certain critical parameters that tend to produce jolts. Limiting the axial speed, however, impairs the productivity of the machine.
The reaction forces produced by the drive train can cause oscillations in the machine which can make it impossible to maintain the desired precision. In this case, the axial velocity should be decreased, or the machine design should be changed to mechanically strengthen the machine. As mentioned above, reducing the axial speed lowers the productivity which is undesirable in most situations. Increasing the mechanical rigidity of the machine can also be quite expensive and therefore negate the advantages originally envisioned for the machine. More particularly, mechanical reinforcement may not even be an option when the mechanical load exceeds a predetermined value.
German Pat. No. DE-A-198 10 996 describes a method for keeping away from the machine base the recoil moment generated by directly driven servo axes. As described in the reference, a direct drive system always produces two forces, namely the desired thrust which drives the machine carriage, and the reaction force produced as a result of the thrust. Both forcesxe2x80x94the thrust and the reaction forcexe2x80x94are always produced simultaneously, have always the same magnitude and are oriented in opposite directions. However, both forces can act on different locations and/or machine elements. This arrangement reflects the physical law of momentum conservation. The machine operator desires only a single force, namely the drive force, which accelerates a machine carriage along the path contour. The second force, the reaction force, cannot be eliminated due to the laws of momentum conservation, but does not contribute to the machine operation and is therefore wasted. German Pat. No. DE-198 10 996 teaches a method for diverting the reaction force before it reaches the machine base, which can still produce undesirable effects. The method described in the reference does not alter the control functions and is therefore limited to direct drives, such as linear motors, which operate without a mechanical transmission or idlers.
It would therefore be desirable to prevent transmission of vibrations which are produced as a result of the reaction forces by the fast axial drives to the machine base. It would also be desirable to It would be desirable to operate the machine at a high linear speed without causing the machine to oscillate.
According to one aspect of the present invention, a drive for a useful load in an machine includes a motor and a drive assembly having at least a first and a second drive element which are in mutual engagement for moving to the useful load. The first drive element is associated with the useful load, whereas the second drive element is connected to a stationary base. The second drive element is movably supported on the base. In this way, the reaction force can be decoupled from the machine base even if the drive system is not a direct drive. In other words, the recoil momentum can be decoupled from the machine base even when using the more common spindle drives, belt drives and rack and pinion drives.
Embodiments of the invention may include one or several of the following features. The first drive element can be connected with a machine carriage, and the second drive element can be connected to a reaction carriage that is supported on the machine base. The drive may also include a first positioning measurement device for measuring the position of the machine carriage relative to the reaction carriage, which can be resiliently supported on the machine base, and a second positioning measurement device for determining the position of the reaction carriage relative to the machine base. One of the positioning measurement devices may include an angular encoder associated with a servo motor.
Moreover, the system may include a control system for controlling the position of the useful load, wherein the characteristic frequency of the reaction carriage when oscillating in the direction of motion is smaller than the control frequency of the control system. The characteristic frequency should be at most 400 Hz, preferably less than 100 Hz, and more particularly less than 30 Hz. The drive element may include a pinion and a toothed rod, a spindle and a spindle nut, a belt pulley and a drive belt, or a pinion and a drive chain.
According to another aspect of the invention, a method for controlling the drive system includes presetting setpoint position data for the useful load and correcting the setpoint position data with actual position data reflecting the position of the machine carriage relative to the machine base. In particular, the method includes defining a setpoint value for a position of the machine carriage relative to a machine base, and adding a correction signal representing a displacement between the reaction carriage and the machine base to the setpoint value, thus providing a corrected setpoint value. The method further includes measuring an actual position value of the machine carriage relative to the reaction carriage, subtracting the actual position value from the corrected setpoint value, thereby forming a differentiated velocity setpoint signal, and measuring the velocity of the machine carriage relative to the reaction carriage by differentiating the actual position value. The velocity of the machine carriage is then subtracted from the differentiated velocity setpoint signal, and a control signal is fed back to a motor that controls the position of a useful load to make the difference between the velocity of the machine carriage and the differentiated velocity setpoint signal equal to zero.